Storage tube and target therefor



@y 9, 1950 A. Ross 2,506,742

STORAGE TUBE AND TARGET THEREEOR Filed June 14, 1947 5E Q\ Si* 4% Y@ S a Q Q?- Qr* u san/w ,mag/v; uw ad wu/yay ya gwn/J INVENTOR.

Patented May 9, I1950 Albert Rose, Princeton, N. J., assignor -to Radio Corporation of America, a corporation of Dela- `ware Application June 14, 1947, Serial No. 754,;63'3

r Claims. (Cl. Z50-453) For -many purposes 't is Vdesirable to record I signal information at some arbitrarytime and in some arbitrary orderat a later time to reproduce copies of such information either in the same or a different order. Teleran radar systemis oneexample of this. Recording on photographic lm is an example, which is substantially manual, of a method of accomplishing this result. A picture once photographed and developed may-be reproduced many times and, if desired, in vpiecemeal and in any order of the various partsof the recorded information. Photography, however, is slow compared with possible methodsutilizing electronic devices. The usual storage type of television pick-up tube is an example of an electronic device satisfying all the conditions enumerated except one: that of reproducing many copies of the recorded information. The usual pick-up tube is designed to reproduce only one copy as the pick-up target of the cathode ray beam `is "wiped clean by one scansion so as to be ready for recording new information.

It is an object of this invention to so modify a television pick-up tube that a single scansion can remove only a fraction of the information recorded on the target with removal of further fractions which are substantial duplicates of the original information until all of the information is removed.

Another object of the invention is to correlate the parts of the tubefand their operations in such a way that each of the repeated scansions of the target will produce a faithful reproduction of Athe information.-

Other objects of the invention will lappear in the following specification, reference being had to the drawing in which:

Fig. 1 is an axial section of a cathode ray beam tube illustrating the principles ofthe invention;

Fig. r2 is a detail illustration of one form of the target; and

Fig. l3 is a vgraph illustrating the invention.

-In a television pick-up ltube the scanning beam either in itself or by its eiects at the target, prorvides an equivalent resistance pathY over which the charge on the target leaks oit. This equivalent resistance R andthe capacitance C of the target, that is, the capacitance between the charged surface and the signal plate, determine the time constant RC of the system.

In the usual storage type television pick-up tube the time constant Tf-RC isfmade approximately equal to the frame time, that is, the time of one scansion. In my invention the tube is so designed that the'tfme constant T is equal to many frame times. This may be done either by increasing the equivalent resistance R or the capactancev C or both. The particular requirethe principles of The present known ments and design of the tube dictate which of these parameters is increased and in what manner.

YFor example, in the orthicon Vpiclr-up'tube in which the electrons of the beam landing on the target have substantially-zero electron velocity, the equivalent resistance R can be increased by decreasing the beam current. This, however,will be found unsatisfactory for two reasons. First, the amount of charge an orthicon target can store for a reasonably small potential swing, required for good resolution, i's not suicient (for a number of separate reproductions. Second, 'the majority vof the reproductions will not be faithful to the original reproduction but will be a black and white approximation, lacking the half-tones of the original. This latter arises from the fact that the potential swing on the target is large compared with the velocity range of the electrons emitted by the thermionic cathode. Under these conditions all the beam 'electrons land on both gray and white charged areas of the target. Hence, a signal from a lgray area isthe same as the signal from a white area.

I have succeeded in obtaining the desired result by use of a special target in which the capacitance C is so high that large amounts vof charge may be stored at relatively small potential. Th'e voltage produ'cedby the charge is and it will thus be clear that the potentialswing may be kept low with this capacity increase. In'

this way I am able to coniine the potential swings of the target areas to such low values that the potentials are much less than the velocity range Particular values of parameters may be computed as follows: t

A low velocity scanning beam such as used in the orthicon has an equivalent resistance given approximately by:

-ihdx 10--12 farads where A is area in square centimeters of the target k is the dielectric constant of thetarget and d is the thickness of the target in centimeters.

3 To obtain one hundred reproductions of the information stored in the target with a frame time of 1/30 of a second the time constant must be:

(3) T=RC= seconds Assuming a beam signal current I of -a amperes, a Value of 5 for 1c and 20 om!l for A the thickness of the target is found by substituting these values and those of (l) in (3) thus:

from which:

d=3 10-5 cms.

This is about one one-hundredth of the thickness of the target of the usual orthicon pick-up tube for normal television purposes. One may obtain a target of this order of thinness, for example, by cutting a section from a bubble blown from molten glass as disclosed in my application led November 28, 1945, Serial No. 631,441, except that a glass of higher resistivity should preferably be used such as G705BA glass of the Corning Glass Co.

I take a section I of the film from a glass bubble having the required area and mount it on a glass ring 2 of uranium glass (Fig. 2) by heating the ring and lm until the glass film fuses to the ring and becomes suiciently plastic to stretch taught thereacross by surface tension as at 3. The ring and attached film is then cooled to room temperature. Fig. 2 is, of course, not drawn to scale, the film thickness being greatly exaggerated.

The glass used to produce the bubble from which the thin film, section I, is cut to make the target 3 should have a resistance ranging from 1012 to 1015 ohms per cubic centimeter which would comprise not only G705BA glass but also G705B, GTO'IDG, G790I-I, G'72OGO, G750AH, G79OJ and various other glasses.

The target glass 3 after being mounted in the frame or ring 2 is coated with a conducting transparent film 4 in known ways (Figs. 1 and 3). It is then mounted in the tube envelope, evacuated and treated in the usual known manner to form the photocathode mosaic 5. This may be of the caesiated silver-silver oxide form but any other suitable mosaic may be used.

A target made in the manner described above will have the desired order of thinness. For such a target, the capacity between the photomosaic cathode 5 and the conductive lm 4 may be approximated as follows from the data given above:

The time factor RC is given as 3.

From Equation 1,

Rw ohms and where Il-8 amperes,

RM10'I then C-3 10-'1 farads C-300,000 pufarads This order of capacity can be compared with the 2,000 ppfarad capacity of a mica target in the usual orthicon pickup tube used for normal television purposes. Such a mica target has a thickness in the order of 3 to 5 10-3 centimeters.

It is desirable to use electron multiplication of the signal and for this purpose I prefer the 4 multiplier type disclosed in the application of Paul K. Weimer, filed Sept. 16, 1944, Serial No. 554,494, now U. S. Patent No. 2,433,941, issued January 6, 1948, in which the end of the first anode 6 constitutes the rst dynode. A plurality of additional dynodes are arranged in succession around the rst anode generally indicated by the block diagram 1. The output terminal of the multiplier is shown at 8, the intermediate voltage terminals of the multiplier not being shown. Reference is made to thatl application for the details of the multiplier. An anode ring 9 known as the persuader has the same applied positive voltage as the anode but it may vary somewhat in some cases. This electrode aids in producing a field that directs the secondary electrons into the second dynode of multiplier stages I as indicated at I0. Either the well known electrostatic or electromagnetic deiiecting means may be used, electromagnetic being shown at H comprising coils that produce fields at right angles to each other and to the axis of the tube. These coils would be connected to the usual saw-tooth or other variable voltage sources of line and frame frequency, not shown. A wall coating anode I2 or equivalent is positioned between the persuader and the target bearing the photocathode 5. Either the well known electrostatic or magnetic means l may be used to focus the beam B on the target and the return beam B on the first anode 6. I have shown a direct current solenoid coil I3 by way of example for producing a magnetic focusing field.

The signal plate 4 and the cathode I4 will be connected to the minus terminal B1 of the voltage supply and the grid I5 would be connected to a somewhat more negative terminal Ba The voltage applied to the various electrodes as well known to those skilled in the art and the positive and negative terminals are designated by reference characters -I- or B with subscript numerals to indicate that the potentials may be different. Suitable potentials for these electrodes are given in said Weimer application.

In the operation the teleran or other suitable signals produce a bright fluorescent ash of very brief period and a less bright decaying phosphorescence on the phosphor of the receiving scope I6. When this is observed directly in the radar scope the bright flash has little utility as an indication because it is of such brief duration. To spread the effect out over a sufficiently long period and to store the signals for reproduction at any later time the light from the scope I6 is focused on the photocathode mosaic as indicated by lens I'I. Due to the extreme thinness of the target such as 3 10-5 cms. calculated above, a charge is produced on the target that is removed only by repeated scansions. For each scansion the electrons land in sufficient quantity to reduce the charge l/ 10o of the total charge, the beam current I being shown to have this value. This modulates the beam B' which bombards electrons from the first dynode 6 and the secondary electrons are attracted into the second stage of the multiplier shown in block diagram at 1. Secondary electrons are bombarded from the various stages thereof (not shown) in succession as explained in said Weimer application so that a greatly multiplied signal may be taken off the output lead 8 and additionally amplified if desired, but this is not shown.

The operation may be explained as follows:

Referring to Fig. 3 the energy distribution of beam electrons emitted from the cathode I4 of Fig. 1 is indicated by the graph. With no light on the target, beam electrons will land until the insulation target drops to substantially V3 volts belows the potential of cathode i4 from which they came. Now, if a photoimage is formed on the target from the scope i6 photoelectrons will be emitted and a charge image will be formed thereon which establishes a potential pattern thereover. When the beam is directed at black areas, substantially no electrons will land, the potential being around V3. When the beam reaches gray areas, having a positive potential V2, electrons with velocities above V2, that is, velocities V2 to V3, Wili land and after many scansions bring these areas down to V3. When the beam is over White areas having a positive potential of V1 volts, all electrons having velocities above V1, that is, velocities Vi to V3, will land and after many scansions bring the potentials to V3. The number of electrons landing for gray areas is the summation of the ordinate values from O to Q2 and the number for White areas is the summation of the ordinate values from O to Q1. Thus, the return beam that goes into the multipliers is correctly modulated by the photoimage produced by the light coming from the radar scope I6. In making this image the bright fluorescent flashes are utilized in producing the charge image.

If one were to use the ordinary low capacity target and attempt to reduce the beam current to increase the time constant, that is, increase R instead of C in the time constant RC, the potential swing would have been greater than the velocity range, say V5 for gray areas and V4 for white areas. The number of electrons landing on gray areas would then be the same as those landing on white areas, being the summation from O to Qu in each case. Thus, only a black and white picture could be produced with no intermediate shades.

Instead of a glass film target other forms may be used such as the precipitated silica layer disclosed in the application of Stanley F. Forgue, led December 16, '1946, Ser. No. 754,715.

Various modications may be made in the improvement without departing from the spirit of the invention.

What I claim as new is:

1. A cathode ray pickup target comprising a transparent dielectric film having a thickness of the order of 3 105 centimeters, a transparent conducting coating on one side or" said nlm and a photosensitive mosaic on the other side thereof.

2. A cathode ray pickup target comprising a iilm of glass having a resistivity of 1012 to 1015 ohms per cubic centimeter and a thickness of the order of 3 105 centimeters, a transparent conducting coating on one side of said film and a photosensitive mosaic on the other side thereof.

3. A cathode ray pickup target comprising a frame, a lm of glass extending across said frame and attached thereto, said glass having a resistivity of the 1012 to 1015 ohms per cubic centimeter and the lm being of the order of 3 1r5 centimeters thick, a transparent conducting coating on one side of said lm and a photosensitive mosaic on the other side thereof.

4. An electron discharge device comprising, an electron gun structure including a cathode elec trode for producing a beam of electrons along a path, a target electrode including a thin dielectric sheet positioned transversely to said beam path, a photosensitivc mosaic on the suri'ace of said dielectric sheet facing said electron gun and a conductive film on the other surface of said dielectric target sheet, conductive means for providing said cathode electrode and said conductive nlm with a common potential, said dielectric lm having a thickness in the order of 3x10-5 centimeters whereby changes in potential of said mosaic relative to said conductive film are less than the normal velocity range of said electron beam.

5. An electron discharge device comprising, an electron gun structure including a cathode electrode for producing a beam of electrons along a path, a target electrode including a thin dielectric sheet positioned transversely to said beam path, a photosensitive mosaic on the surface of said dielectric sheet facing said electron gun and a conductive nlm on the other surface of said dielectric target sheet, conductive means for providing said cathode electrode and said conductive lm with a common potential, said dielectric lm having a thickness providing a capacity in the order of 300,000 ppfarads between said photosensitive mosaic and said conductive iilm.

ALBERT ROSE.

REFERENCES CITED The following references are of record in the le of this patent:

UNITED STATES PATENTS Number Name Date 2,213,174 Rose Aug. 27, 1940 

